1. Field of the Invention
The present invention relates to an electric circuit breaker, and more particularly to such a circuit breaker of the type suitable as a substitution for a conventional fuse holder.
2. Description of the Prior Art
In the field of the electric circuit breaker, there has been a known practice to encase a breaker assembly in a housing shaped substantially cylindrically and dimensioned selectively so that the circuit breaker has the same external appearance as a conventional elongated fuse holder. Prior breakers of this type can be found in U.S. Pat. Nos. 4,068,203, 4,123,737, 4,363,016 and 4,518,943, each of which discloses the use of a snap-acting bimetallic strip carrying thereon a movable contact in cooperation with a spring loaded insulation screening board. The insulation screening board is latched directly by the movable contact in closed position against the spring bias in the normal operating condition. The insulation screening board is unlatched to move between the separated contacts for insulation therebetween in response to fault current condition. In such prior breakers, since the snap-acting bimetallic strip is self-biased in the direction of closing the contact and operates to latch in the screen board against the spring bias by engaging the movable contact thereon with the screening board, the bimetallic strip receives constant spring force through the screening board which may distort or at least upset the heat responsive characteristics of the bimetallic strip after an extended service life, eventually resulting in unreliable fault-current responsive tripping. This poses a critical problem for the breaker, in addition to that the screening board itself makes the assembly of the breaker complicated. Another prior circuit breaker is shown in U.S. Pat. No. 4,048,608 in which a contact actuator is latched by a bimetallic strip formed separately from the contacts. However, the contact actuator is operatively connected to a movable contact through a rather complicated and space-consuming mechanism including an eccentric coupling, which inhibits the breaker from being made compact enough to be substituted for the conventional fuse holder.